Known substrates for flexible printed wiring boards are manufactured by bonding an insulator made of a nonthermoplastic polyimide film and a conductor to each other through the intermediary of an epoxy resin adhesive, an acrylic resin adhesive or the like. Known among such substrates are a substrate for a three layer-structured single-sided flexible printed wiring board (hereinafter referred to as “single-sided substrate”) in which substrate a conductor is disposed on one side of an insulator through the intermediary of an adhesive layer, and a substrate for a five layer-structured doubled-sided flexible printed wiring board (hereinafter referred to as a “double-sided substrate”) in which substrate two conductors are each stacked on one of both sides of an insulator through the intermediary of an adhesive.
However, the known substrates for flexible printed wiring boards have problems in that in both the single-sided substrate and the double-sided substrate, an adhesive layer is present between a conductor and an insulator, and hence the heat resistance, flame retardancy, electric properties and the like are degraded. Additionally, there is a problem that the rate of dimensional change is large when the conductor is etched and the substrate is subjected to some type of heat treatment, leading to troubles caused in the subsequent processes.
For the purpose of solving the above described problems, as for the single-sided substrate, there has been developed a method in which the nonthermoplastic polyimide resin layer is formed directly on the conductor without any adhesive layer between the conductor and the insulator (for example, JP-A-60-157286, JP-A-1-245586, JP-A-4-274382 and JP-A-8-250860).
On the other hand, as for the double-sided substrate, there has been proposed a method in which the above described problems are intended to solve by using a thermoplastic polyimide capable of thermocompression bonding for an adhesive layer for the purpose of adhering the nonthermoplastic polyimide film and the conductor to each other (for example, JP-A-2000-103010, JP-A-2001-270033, JP-A-2001-270034, JP-A-2001-270035, JP-A-2001-270037 and JP-A-2001-270039).
However, in the configuration, it is not the nonthermoplastic polyimide but the thermoplastic polymer that is directly in contact with the conductors, so that the rate of dimensional change becomes large when the substrate is subjected to some type of heat treatment, and consequently the configuration cannot sufficiently solve the above described problems.